const canvasDiv = document.querySelector('#CanvasDiv');
const imageArr = ['image/1.jpg','image/2.jpg','image/3.jpg'];

canvasDiv.style.width = window.innerWidth + "px";
canvasDiv.style.height = window.innerHeight + "px";

canvasDiv.style.backgroundImage = "url(" + imageArr[ Math.floor( Math.random() * imageArr.length ) ] + ")";